Such boosters are well known in automobile technology and comprise in general an envelope within which is disposed a piston formed from a hub and a skirt. The piston defines a front chamber connected permanently to a source of low pressure and a rear chamber connected selectively to the front chamber or to a source of high pressure by a valve means. This valve means is actuated by a control rod which is capable of bearing, through the intermediary of the front face of a plunger, on the rear face of a reaction disc which is solid with a thrust rod, the thrust rod actuating a master cylinder. The valve means usually comprises a valve member whose annular front face co-operates with a first circular valve seat formed on the plunger and a second circular valve seat formed on the piston and of diameter greater than that of the first valve seat. A valve spring urges the annular front face towards the valve seats so that the front face is always in contact with at least one of these two seats.
Such boosters present several disadvantages. Thus, in order to ensure that communication between the front and rear chambers of the booster is closed off before opening communication between the rear chamber and the source of high pressure, it is necessary to manufacture the plunger and the piston hub with very tight tolerances to avoid the control rod having too long a dead stroke, or even to design the valve means in such a way that the "valve lift" between the valve and the first valve seat will be as small as possible.
An additional disadvantage of the known boosters resides in the fact that the only radial passage opened to the high pressure air towards the rear chamber during actuation as well as the only axial passage opened to the air from the rear chamber towards the front chamber during release of breaking, produce turbulence in the moving air, slowing the action of the booster and inducing operational noises which can become troublesome.
Another disadvantage follows from the concentric and substantially coplanar disposition of the two valve seats, which means that the flexible membrane of the valve member is subjected to a variable pressure differential between the high pressure which is substantially constant prevailing permanently within the tubular hub around the input rod, and the variable pressure prevailing in the annular chamber surrounding this part of the flexible membrane of the valve member, on one part of which is exerted the pressure prevailing in the front chamber of the booster and on another part of which is exerted the variable pressure prevailing in the rear chamber of the booster.
This pressure differential, which exists in the rest condition and in the phase of brake release, applies an axial force to the annular front face of the valve member which adds to the valve spring force and which the plunger of the valve must overcome during each phase of brake release to disengage the annular front face of the valve member from the first valve seat formed in the hub and re-establish the communication between the rear working chamber of the booster and the vacuum chamber, which requires the return spring of the input rod to be oversize and in particular results in the driver having to apply a high force to bring the booster into operation, this force being known in the art under the term of "step-in force".
In addition, as the valve member is formed of a flexible diaphragm fixed sealingly by its outer peripheral edge to the piston by means of a metallic cup which also serves as load bearer for the valve spring and for a return spring of the control rod, the assembly of the three way valve means in the piston hub is a delicate and complex operation, with a rejection rate which is not negligible and which causes a significant increase in the cost of the booster.
The document DE-A-3 445 118 attempts to offer a solution to these problems, by making the plunger in a design similar to the spool of a slide valve. The plunger is capable of sliding within a bore in the hub of the piston to close off or open two windows formed in the hub communicating with the rear chamber of the booster. Such a design still requires a very precise match between the bore of the hub in the piston and the plunger sliding in the hub. It also requires the presence of a seal member at the edges of one of the windows formed in the hub of the piston and of a seal member at the end of the plunger. These seals cooperate either with a rib on the plunger, or with the edge of one of the windows, and they are therefore liable to deteriorate very rapidly, which is harmful for the reliability of the booster they are fitted in. In addition, this document still only provides a single axial passage between the front and rear chambers, and a single radial passage between the high pressure source and the rear chamber, resulting once again in turbulence which generates operating noise and in reduced speed of operation of the booster